The goal of the present research proposal is to understand structure/function of the RNA Recognition Motif RRM), one of the most common protein families in eukaryotic genomes. By understanding how proteins belonging to the RRM superfamily recognize RNA, a new level of understanding of the molecular determinants of specificity, a key biological property of this important protein family, will be provided. The fundamental knowledge generated will be exploited through our long-term objective of designing RRM proteins with new specificity that control gene expression in a directed way. Such proteins would provide new tools and avenues for the control of gene expression and could have wide applicability in basic research and biomedicine. A second aim is to dissect the structural basis of control in a key step in mRNA biogenesis, the formation of the mature 3'-end by cleavage and polyadenylation, a fundamental aspect of the biology of all eukaryotes. Since 30% of all human mRNA undergo alternative 3'-end-processing events generating molecular species with distinct biological properties and gene expression profiles, understanding how mRNA biogenesis is controlled is critical to understanding many cellular decisions and the origin of several human diseases. Through this research program, a number of questions and hypothesis on how the RRM functions in regulation of RNA biogenesis will be addressed by executing five specific aims that will i. Provide a detailed analysis of how the sequence of an RRM dictates its RNA-binding specificity RNA through a new experimental approach; ii. Analyze dynamic processes which are key to understanding RNA recognition; iii. Evolve an RRM with new specificity and function; iv. Analyze at the molecular level a gene-specific nuclear autoregulatory circuit that function at the level of RNA 3 '-end processing and v. Analyze in structural detail a general mechanism by which mRNA 3'-end processing is controlled during cellular development and differentiation and other biological processes.